The present invention relates to a method of adjusting a frequency response in a microwave stripline filter device which may be used as a band-pass filter for example.
A variety of microwave stripline filters that can be used as bandpass filters for microwaves are known in the art.
FIG. 1 of the accompanying drawings illustrates a microwave strip line filter of a known type which comprises a pair of dielectric substrates 1a and 1b made of dielectric ceramic material having a high dielectric constant and a lower dielectric loss such as BaO-TiO.sub.2 or BaO-TiO.sub.2 -rare earth or the like, the dielectric substrates 1a and 1b being stacked to each other. The dielectric substrates 1a and 1b are provided with external ground conducting layers 2a and 2b on the peripheral portion and bottom surface thereof, respectively. On the upper surface of the lower dielectric substrate 1a are disposed a plurality of stripline resonator conducting layers 3a which operate as a filter element. Each resonator conducting layer 3a has one end connected to the ground conducting layer 2a to form a short circuit end, and the other end or an open circuit end spaced from the ground conducting layer 2a. The open circuit ends of the respective resonator conducting layers 3a are alternately disposed so as to form an interdigitated configuration. The upper dielectric substrate 1b is fixed on the lower dielectric substrate 1a, and the ground conducting layers 2a and 2b of the respective dielectric substrates are connected to each other. One example of such arrangements is disclosed in U.S. Pat. No. 4,157,517.
It is known that the response frequency of the stripline filter device of the above described type depends upon the dielectric constant of the used dielectric substrates and the dimensions of the respective resonator conductors. Upon the manufacturing of the filter devices the dielectric constant of the substrates and the dimensions of the resonator conductors are rigorously controlled. However, the manufactured filter devices inevitably show variations in terms of these factors and require an operation of frequency adjustment after the pair of dielectric substrates are assemblied.
The stripline filter device as illustrated in FIG. 1 is designed to initially have a resonance frequency lower than a desired value and after assemblying the pair of the dielectric substrates 1a and 1b to form the filter, the external conductor or ground conducting layer 2b provided on the upper surface of the upper substrate 1b is partially removed at regions 4 adjacent the open circuit ends of the resonator conducting layers 3a to reduce the stray capacitance between the external conducting layer 2b and the respective resonator conducting layers 3a thereby increasing the response frequency of the filter to the desired value.
With this adjusting method, however, the frequency response may be deviated again when the assembled filter body is contained in a casing after the adjustment of the frequency response is made as the removed regions 4 are brought to contact with or close to the upper inner wall of the outer casing so that the stray capacitance may be changed from the adjusted value.
In order to solve this problem, an attempt has been proposed in Japanese Patent Kokai No. 1-251801. According to a method of frequency adjustment disclosed in this reference, in a stripline filter in which a pair of dielectric substrates are stacked together with a plurality of resonance conductor strips arranged therebetween, openings are formed on the lateral sides of dielectric substrates at the positions facing the short circuit ends of the resonance conductor strips for adjustment of the response frequency.
Another solution for the problem has been proposed in Japanese Patent Kokai No. 2-292901 which discloses a method of frequency adjustment for a microwave stripline filter having a pair of dielectric substrates to be stacked, each provided on the outer surface with a ground conductor, together with a plurality of resonator electrodes arranged on the inner surface of at least one of the dielectric substrates, one end of each resonator electrode being connected to the the ground conductor to form a short circuit end, and the other end or an open circuit end being spaced from the ground conductor, wherein the ground conductors are partially removed at the locations on the lateral sides of the dielectric substrates facing the open ends of the resonator electrodes, at the locations connected to the short circuit ends of the resonator electrodes and at the locations where notches are formed on the lateral sides of the dielectric substrates, the notches facing the open ends of the resonator electrodes.
A further solution has been proposed in Japanese Patent Kokai No. 1-219580 which discloses a method of frequency adjustment for a stripline filter having a pair of dielectric substrates stacked together with a plurality of resonance conductor strips arranged therebetween. According to this method ground conductors are provided on the outer surface of each of the dielectric substrates except the lateral sides, the short circuit end of each of the resonator conductor strips is extended as far as one of the ground conductors along the lateral sides, the open end of each of the resonator conductors is removed together with a portion of the corresponding lateral side of either of the corresponding dielectric substrate, and a piece of corrective conductor is provided on the open circuit end of each of the resonator conductors.
The known methods of frequency adjustment as mentioned above have disadvantages. Firstly, while the method disclosed in Japanese Patent Kokai No. 2-251801 can reduce the response frequency of the filter by forming the openings on it for adjusting the response frequency, it can not raise the frequency once it is made too low. Besides, the proposed method involves cumbersome operations.
While the method disclosed in Japanese Patent Kokai No. 2-292901 can raise the response frequency of the filter, the extent to which it can adjust the frequency is rather limited and the effect of the frequency adjustment becomes poor when the open circuit end of each of the resonator conductors and the corresponding ground conductor are separated from each other by a considerable distance.
With the method disclosed in Japanese Patent Kokai No. 1-219580, the response frequency of the stripline filter can be increased to a considerable extent. However, the profile of the filter needs to meet certain given requirements if the method works effectively.